Forced flow vapor generator



Dec. 7, 1965 M. WIENER 3,221,713

FORCED FLOW VAPOR GENERATOR Filed Aug. 20, 1963 s Sheets-Sheet 1 INV ENTOR.

Murray Wiener BY 6 ATTORNEY Dec. 7, 1965 M. WIENER FORCED FLOW VAPORGENERATOR Filed Aug. 26, 1963 3 Sheets-Sheet B FIG.2

Dec. 7, 1965 M. WIENER FORCED FLOW VAPOR GENERATOR 3 Sheets-Sheet 5Filed Aug. 20, 1963 H 3 9 7 h hu h l w H UIHIHIHHIIWHIHW h l' s 1 un l vl m m m w v ...\w m F A) w 2 v m I /m L k r k 2 n. i A IIM. 3L m.

United States Patent 3,221,713 FORCED FLOW VAPOR GENERATOR MurrayWiener, Akron, Ohio, assignor to The Babcocir & Wilcox Company, NewYork, N.Y., a corporation of New Jersey Filed Aug. 20, 1963, Ser. No.303,299 4 Claims. (Cl. 122-406) The present invention relates in generalto the construction and operation of a forced flow fluid heating unitand more particularly to improvements in the construction andarrangement of fluid heating circuits especially adapted for use in aforced circulation once-through vapor generating and superheating unit.

The invention herein disclosed is an improvement over the vaporgenerating unit described in US. Patent No. 3,125,995, issued March 24,1964 in the name of P. H. Koch. The general object of the presentinvention is the provision of a fluid heating unit of the characterdescribed so constructed and arranged as to produce superheated vaporfrom a vaporizable fluid over a desired range of high pressures andtemperatures; to assure an optimum quantitative and qualitativedistribution of fluid to all fluid fiow paths; and to assure an optimumrelation of fluid velocity within the tubes to heat input into the tubewalls to effect adequate cooling, thereby maintaining the tube walls ata safe operating temperature.

The construction of forced circulation once-through steam generatorsrequires the use of a large number of parallel circuits connectedbetween inlet and outlet headers. One of the fundamental problemsinvolved with such a steam generator is the control of the flow throughthe various parallel circuits in order that the flow in each circuitwill be stable and the enthalpy of the fluid discharged from anyindividual circuit will be close to the average of that from allcircuits, in which case the circuits will be in a balanced flowcondition. Unbalanced flow may be caused by unequal heat absorption inparallel circuits due to unsymmetrical arrangement of heating surfaces,slag accumulation, or part-load operation with one or more burners outof service; or may be due to unequal resistances caused by differentlengths of circuits. When steam or water, or mixtures thereof, is heatedin parallel flow paths provided by the furnace wall tubes or tubularpanels disposed in the furnace, unbalanced heat and/ or fluid flowdistribution may lead to excessive localized tube metal temperaturesand/or to excessive temperature differentials between adjacent furnacewall tubes, resulting in excessive thermal stresses in the furnacewall-forming components. The problem of unequal fluid flow distributionis accentuated when the fluid supplied to the high heat absorbingparallel flow circuits of the furnace is a mixture of steam and water.Whenever a steam-water mixture must be distributed to many tubes ofparallel circuits, the possibility of separation of the steam-watermixture exists. Thus, one tube may receive saturated steam and anothertube may receive saturated water or any combination of the twocomponents. Such a condition imposes a limit on the rate of heatabsorption that the tubes of the parallel flow circuits can toleratewithout exceeding allowable metal temperatures or allowable temperaturedifferences between adjacent tubes.

The fluid distribution system described in the above identified US.patent has proven to be successful for its intended purpose of uniformlydistributing circulating fluid both quantitatively and qualitatively, toa plurality of parallel-flow fluid heating circuits. However, it hasbeen found that, due to uneven heat absorption in the individual fluidheating circuits, uniform quantitative distribution of fluid to thecircuits tends to produce exorbitant localized tube metal temperaturesand exhorbitant temperature differentials between adjacent furnace walltubes under certain operating conditions. For example, it has been foundthat the heat absorption in the circuit having the highest heatabsorption rate is about fifty percent higher than in the circuit havingthe lowest heat absorption rate when both circuits have equal amounts offluid flowing therethrough. To remedy this situation and to effectbalanced conditions whereby the enthalpy of the fluid discharged fromany individual circuit will be close to the average of that from allcircuits, it has become necessary to provide flow restrictors in theindividual circuits so that the flow through each may be adjusted. Itshould be recognized that the adjustment of flow in this instance refersonly to the quantitative and not the qualitative characteristics of theflow, i.e., it is still desirable that the enthalpy or quality of thefluid flowing into all the circuits be uniform.

To obtain the necessary flow adjustment, a variable orifice valve hasbeen installed in each circuit and set to effect the necessary pressuredrop. A pressure drop of about psi. was found to be required across thevalve in the circuit requiring maximum restriction, i.e. the circuithaving the lowest heat absorption rate. After a short period ofoperation with the valves in service, it was noted that the flow in thecircuits was gradually changing, and that temperature limitations insome tubes were being exceeded. It was also noted that the changes inflow were most notable in those circuits having the greatestrestriction, and that the fiow in the unrestricted circuits wasrelatively unchanged. Inspection of the valves revealed a deposition onthe seats and discs of the variable orifice valves projecting into theflow stream and restricting flow therethrough.

It was known from experience that plate-type or sharp edge orifices,when used in boiler fluid flow circuits, tend to accumulate deposits onthe upstream side of the orifice. In time, these deposits extend overthe orifice edge and restrict the free flow area. Analysis of thesedeposits indicates that they are composed primarily of metal oxides andthat they are of such a character as to be extremely resistant tomechanical or chemical removal efforts. It has also been found that theamount of deposit was generally proportional to the pressure dropthrough the orifice and the abruptness with which the pressure drop isexperienced. Those restrictors having the greatest pressure drop and themost abrupt pressure change were found to be the most susceptible to theformation of the above described depositions. These phenomena previouslyexperienced with depositions on platetype orifices were obviously thecause of the deposition problem in the variable orifice valves.

So a further and more specific object of this invention is to provide adevice and system for selectively restricting the flow in the individualfurnace wall heating circuits so that balanced enthalpy conditions maybe obtained at the discharge ends thereof. This aspect of the inventionis embodied in a forced circulation fluid heating unit having a furnacechamber supplied with high temperature heating gases, with each wall ofthe furnace including a row of upwardly extending contiguous upflowtubes arranged for parallel flow of fluid therethrough relative to eachother and supplied at their lower inlet ends with a vaporizable fluidfrom a common source. These tubes are grouped into a plurality ofparallel flow fluid heating circuits, some of which are arranged toabsorb less heat than others. Provisions are made for distributing thefluid from the common source into a plurality of parallel flow supplypipes or conduits for flow into the upflow tubes of each circuit. Theconduits serving those circuits having relatively low heat absorptionrates are provided with flow inhibitors to compensate for the inequalityof the heat absorption in the circuits and thereby promote substantiallyequal enthalpy conditions at the outlet ends of all the upflow tubes.Each flow inhibitor includes a venturi-shaped flow restrictorconstructed to avoid abrupt changes and to effect gradual reduction ofpressure.

The various features of novelty which characterize my invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages and specific objects attained by its use,reference should be had to the accompanying drawings and descriptivematter in which I have illustrated and described a preferred embodimentof the invention.

Of the drawings:

FIG. 1 is a partially diagrammatic sectional elevation of a forcedcirculation once-through steam generating unit constructed and operablein accordance with the invention;

FIG. 2 is an enlarged representation of the lower portion of the furnaceof FIG. 1 taken along line 2-2 of FIG. 4;

FIG. 3 is a fragmentary front view taken along line 33 of FIG. 4;

FIG. 4 is a fragmentary plan section taken along line 44 of FIG. 2;

FIG. 5 is a sectional view of a preferred type of flow restrictorelement;

FIG. 6 is a plan section taken along line 6--6 of FIG. 1;

FIG. 7 is a plan section taken along line 77 of FIG. 1; and

FIG. 8 is a plan section taken along line 88 of FIG. 1.

In the drawings the invention has been illustrated as embodied in atop-supported forced flow once-through vapor generating and superheatingunit designed for the production of superheated steam at pressures belowthe critical pressure of 3206 p.s.i. The unit herein shown is of thesame general construction and arrangement as the unit disclosed anddescribed in detail in the abovementioned US. Patent No. 3,125,995. Thedetails of the unit will therefore be described herein only so far as isnecessary to recognize the import and applicability of the invention,and reference should be made to the aforesaid US. patent for furtherdetail.

The description of the invention in terms of this particular unit is notmeant to serve as a limitation on the invention, and it should berecognized that the invention may also be used in conjunction with anytype of forced flow once-through vapor generator operating either aboveor below the critical pressure.

The main portions of the unit illustrated include an upright furnacechamber 10 of substantially rectangular horizontal cross-section definedby a front wall 12, a rear wall 14, side walls 16 and a roof 18 andhaving a gas outlet 20 at its upper end opening to a horizontallyextending gas pass 22 of rectangular vertical cross-section formed by afloor 23 and extensions of the furnace roof 18 and side walls 16. Thegas pass 22 communicates at its rear end with the upper end of anupright gas passage 24 of rectangular horizontal cross-section formed bya front wall 26, a rear wall 28, side walls 30 and a roof 32. The lowerportions of the front and rear walls of the furnace slope inwardly anddownwardly and cooperate with the furnace side walls to form a hopper4t) and a rectangular throat passage 42 for discharging ash into an ashpit, not shown.

A secondary superheater 44 is disposed in part in the upper portion ofthe furnace 10 adjacent the gas outlet 20 thereof, with the remainderoccupying the furnace end of the gas pass 22. An intermediate primarysuperheater section 60 and a secondary reheater (not shown) are disposedin the end of the gas pass 22 nearest the entrance to the upright gaspassage 24. The upright gas pass 24 is occupied by a primary superheater58 and a primary reheater (not shown), each of which compriseshorizontally extending nested multiple-looped tubes arranged inlaterally spaced panels. The lower end of the upright gas passage 24 isoccupied by a horizontally arranged multiple-looped return bend tubulareconomizer 66 disposed downstream gas-wise of the primary superheater 58and the reheater. It should be recognized that the above describedsuperheater and reheater sections are interconnected by appropriatepiping (not specifically identified) and that vapor flows seriallythrough these sections in accordance with the description in theaforesaid US. Patent No. 3,125,995.

Combustion air and fuel are supplied to the lower portion of thefurnace. The resulting heating gases flow from the furnace chamber 10over the radiant heat absorbing portion of the secondary superheater 44,and then through the gas outlet 20 and over the convection heatabsorbing portion of the secondary superheater 44 in the gas pass 22.Continuing through the gas pass 22, the heating gases flow over theintermediate primary superheater 60 and the secondary reheater (notshown) and enter the upright gas passage 24 at its upper end and fiowdownwardly therethrough over the primary superheater 53 and primaryreheater and thence through the economizer 66 to outlet duct 67 throughwhich the gases are discharged into an air heater (not shown).

The furnace chamber 10 is fired by horizontally extending burners 68arranged to direct fuel and air in mixing relationship into the chamberthrough corresponding burner ports in the boundary walls of the furnace.Preheated air is supplied to the burners by a forced draft fan (notshown) which discharges air under pressure through the air heater and aduct 70 to a vertically extending windbox enclosing the burners 68 andthe lower portion of the boundary walls of the furnace 10.

The front, rear and side walls of the furnace 10 and the side walls ofthe gas pass 22 are formed in most part by insulation covered fluidheating tubes. In accordance with the present invention, the furnaceboundary wall fluid heating surface is 50 arranged that the distributionof flow to all fluid flow paths is substantially proportional to theheat absorption rates of the particular fluid flow paths; that themaximum temperature differential between the adjacent tubes is below apredetermined critical limit, thereby maintaining differential expansionin the walls within safe limits; that the tube surfaces in differentzones of heat intensity in the furnace are sufficient in quantity tocarry away the heat at a rate adequate to prevent overheating of thetubes; and that the tubes are of suflicient inside diameter along theirlengths to provide adequate fluid circulation velocities. Accordingly,each of the boundary walls of the furnace 10 is lined by a row ofupwardly extending parallel tubes arranged in groups to form coplanarlaterally contiguous radiant heat absorbing tubular panels, or circuits,the front wall 12 having a row of tubes 80, the rear wall 14 including arow of tubes 87, and each side wall having a row of tubes 90. The tubes90 also line the portion of the side walls of the gas pass 22 oppositethe secondary superheater 44. Some of the rear wall tubes 87 have theirupper portions bent inwardly and upwardly to form a nose arch 96; thenrearwardly and upwardly to form the floor 23 of the gas pass 22; andthen vertically to form part of a screen 98 disposed at the rear end ofthe gas pass 22. Intermediate portions of some of the tubes of thefront, rear and side walls in the furnace are suitably bent to form theopenings or ports for the burners 68.

As shown in FIGS. 1 and 7, the front wall 12 includes a multiplicity ofinitial or first pass upflow tubes A arranged in parallel spacedrelation and for parallel flow of fluid therethrough, spaced about atube diameter apart to provide intertube spaces, and extendingthroughout the high heat intensity burner zone of the furnace from thelevel of the top of the hopper to a point about half way between the toprow of burners and the nose arch 96. The tubes 80A have their oppositeend-s connected to horizontal outlet and inlet headers 82 and 84,respectively, disposed outside of the wall 12, while the correspondingtubes of each side wall 16 have their opposite ends connected tohorizontal outlet and inlet headers 92 and 94, and the correspondingtubes of the rear wall 14 have their lower and upper end-s connected tohorizontal inlet and outlet headers 88 and 86, respectively.

Since the tube lengths of the burner zone are exposed to gases of higherheat intensity than those above and below this zone, their totalabsorption is higher and the quantity of heating surface presented bythe tube lengths in the burner zone must be greater than that above andbelow the burner zone to carry away the heat at a rate sufficient toprevent overheating of the tubes. Accordingly, the front wall 12, aswell as the other furnace boundary walls, also includes a multiplicityof second upflow tubes, designated as tubes 8013 for the front wall,extending throughout the height of the furnace, arranged in parallelspaced relation and for parallel flow of fluid therethrough, anddisposed in the spaces between and contiguous to the initial upflowtubes, tubes 80A in the case of the front wall, along the heightthereof, so that the number of tubes presented to the gases in the zoneof high heat intensity is double that above and below this zone, asshown in FIGS. 6 through 8. Thus the tubes 80B are spaced about a tubediameter apart and contact the tubes 80A through the height of theburner zone. The same relation applies to the corresponding tubes of theother upright furnace boundary walls. The tubes 80B have their upper andlower ends connected respectively to horizontal outlet and inlet headers85 and 83 situated outside of the furnace; while the corresponding tubes90B of each side wall 16, as shown in FIGS. 1 and 2, have their oppositeends connected to horizontal inlet and outlet headers 95 and 93, and thecorresponding tubes of the rear Wall 14 have their lower and upper endsconnected to horizontal headers 81 and 89, respectively. The furnaceboundary wall fluid heating tube portions in the zone of high heatintensity are covered by metallic casing suitably secured thereto, whilethe furnace boundary wall tube portions above and below the zone of highheat intensity, these being second upflow tube portions, have theirintertube space-s closed by metallic webs 79 rigidly secured to thetubes, as shown in FIGS. 6 and 8.

For the unit shown, feedwater at a pressure of 3070 p.s.i.g. is suppliedby a feed pump (not shown), to the economizer 66 wherein it is partiallyheated, then flows through a downcomer 76 and supply tubes 77 to theinlet headers 84, 88 and 94, supplying the alternate initial upflowtubes of the radiant heat absorbing fluid heating tubes lining thefront, rear and side walls of the high heat intensity burner zone of thefurnace chamber 10. The water flowing to these headers is in asub-cooled condition, i.e., at a temperature below the saturationtemperature corresponding to the pressure. Having passed through theinitial upflow tubes, the fluid is collected in the outlet headers 82,86 and 92 and passes by tubular connectors 100 to the header 112disposed along the periphery of the gas pass 24 at a positionintermediate the economizer 66 and the primary superheater 58. Theperipheral header 112 is arranged to supply fluid in parallel flow pathsto the tubes forming the baffle walls( not shown) of the gas pass 22 andthe upright gas passage 24 and to the tubes lining the side walls of thegas pass cuit in which the transition of the fluid from a watercondition to a steam-water condition will be located in the secondupflow tubes of the furnace, it is expected that under certain load andpressure conditions the transition will take place before the fluidreaches the second upflow tubes. Thus the fluid supply system for thesecond upflow tubes of the furnace must be so constructed and arrangedas to inhibit. or prevent separation of the steam from the water whenthe fluid consists of a mixture of both. In addition, the system mustpromote mixing of the fluid streams whether they be in a water or asteamwater condition, as they pass from the initial upflow tubes to thesecond upflow tubes of the furnace and thereby provide a substantiallyuniform fluid enthalpy upon discharge to the second upflow tubes. Tocompensate for the inequality of heat absorption in the various portionsor flow circuits of the furnace wall, and to promote substantially equalenthalpy conditions at the outlet ends of the second upflow tubes, it isalso required that the fluid supplied to the second upflow tubes be soproportioned that those tubes or flow circuits having a relatively lowheat absorption rate are supplied with less fluid than those tubes orflow circuits having higher heat absorption rates.

Accordingly, the fluids collected in the header 113 are passed withoutfurther heating downwardly through the upright downcomer 105 extendingalong and outside of the front wall 12 to the spherical fluid mixing anddistributing vessel 107. This vessel 107, as described in detail in saidUS. Patent No. 3,125,995, tends to qualitatively and quantitativelydistribute the fluid to the conduits or pipes 109 which supply fluid tothe headers 83, 81 and 95 serving the second upflow tubes of the furnacechamber walls. The fluid supply system for each of the fluid supplyheaders 83, 81 and 95 comprises one or more supply tubes 109 eachopening at one end to the vessel 107 and arranged to discharge fluid inparallel flow relation to a group of tubes 115 having their outlet endsconnected to the corresponding fluid supply header at uniformly spacedpositions along the length thereof. The tubes 109 are equally spacedabout the periphery of the vessel 107 at their points of connectionthereto and lead radially from the vessel 107 in a common horizontalplane, then extend vertically for distribution of fluid to the tubes115, with the vertical portion of each tube 109 having its upper endcapped by a fluid distribution nipple 117. Tubes 115 of each group leadradially from a corresponding nipple in a common horizontal plane, thenextend vertically for connection to one of the fluid supply headers.Outlet headers 93, and 89 of the second upflow tubes of the furnaceside, front and rear walls are connected by tubes (not shown) for seriesflow of the vapor-liquid mixtures generated in the second upflow tubesto a collecting header 121 from which the fluids pass through a conduit123 to the primary superheater 58.

The particular form of the vessel 107 and the routing and arrangement ofthe conduits leading to and from the vessel assure that a completelyhomogeneous fluid of uniform enthalpy will be discharged to the secondupflow tubes because the fluid streams are intimately mixed as they passfrom the initial to the second upflow tubes of the furnace and becauseseparation of steam from water is inhibited or prevented whenever thefluid is so constituted. Assuming that the back pressure is equal in allof the circuits served by pipes 109, the distributor vessel 107 wouldeffect uniform quantitative as well as qualitative (equal enthalpy)distribution among the supply pipes 109. However, as discussed above,some of the portions of the furnace wall may be subject to more radiantheat than others so that some of the flow circuits supplied through theinlet headers 81, 83 and may have greater heat absorption capabilitiesthan others. In order that uniform enthalpy conditions may be maintainedat the outlet ends of the second upflow tubes, and

in order to avoid exceeding the tube metal temperatures in the tubes ofthe flow circuits exposed to the greatest amount of radiant heat, it isdesirable to proportion the flow of fluid to the various furnace Wallcircuits so that the fluid to a given circuit is substantially directlyproportional to the heat input to that circuit.

By way of example, and not of limitation, the downcomer 105 at 9'' ID.and has a vertical height of about 114 ft.; the vessel 107 is 30" ID.and supplies fluid to thirty-six of the tubes 109, with each tube 109 inturn supplying fluid to four of the tubes 115 which distribute fluid toeleven hundred and thirty-eight second upflow tubes of the furnace byway of the inlet headers thereof. In the preferred construction andarrangement of the fluid mixing and apparatus, tubes 109 should have atleast the equivalent of ten diameters of vertical straight length aheadof the nipples 117 and should have a cross-sectional area sufficient toprovide a minimum fluid velocity of 10 ft. per second. For obviousreasons of economy and simplicity, the proportioning of fluid to thevarious portions of the furnace wall should be effected in the supplyconduits or pipes 109, each of which serves a fluid flow circuitconsisting of a panel of approximately thirty second upflow wall tubes.

The present invention is primarily concerned with the provision of flowinhibiting means in the circuit supply pipes 109. The use of the flowinhibiting means to be described hereinafter as applied to the abovedescribed vapor generator is not meant to limit the use of the flowinhibitors to this particular type of unit, but rather it should beunderstood that the invention concept is applicable to any type offorced flow vapor generating unit, including those of the type operatingat pressures above the critical pressure.

By way of illustration, and not of limitation, in the unit hereindescribed, the estimated required flow through the circuit requiring thelowest flow is 34,000 lb./hr., while the corresponding flow through thecircuit requiring the highest flow is 54,000 lbs./hr. The lowest flowcircuit and the highest flow circuit are shown in FIG. 4 respectivelyidentified by brackets and marked L and H, and the supply pipes 109leading to them are identified as 109L and 109H. In this illustrationthe estimated required flow through the remaining thirty-four circuitsis somewhere between 34,000 and 54,000 lb./hr. To obtain the necessaryrestriction of flow in the circuit L, a flow restrictor tube 200L iprovided in the supply conduit 109L.

The preferred type of flow restrictor tube 200 is shown in FIG. 5 asbeing welded in flow communicating relationship in a supply pipe 109.The flow restrictor tube 200 is constructed in the form of a venturihaving a conical converging inlet portion 201, an intermediateconstricted throat portion 202 and a conical diverging outlet portion203. The internal conical surface of the inlet portion 201 shouldpreferably have an included angle of less than thirty degrees, and thecorresponding surface in the outlet portion 203 should preferably havean included angle of less than fifteen degrees, as is the practice inthe application of venturi flow meters. The internal diameter and thelength of the throat portion 202 are to be sized to provide the pressuredrop required to obtain the desired flow inhibition; however, theinternal diameter of the throat portion 202 should preferably not besmaller than A of the internal diameter of the supply pipe 109 in whichit is interposed.

It is known that the solubility of solids in fluids such as boiler wateris a function of pressure, and that a change in pressure tends tofacilitate precipitation of the solids from solution. There is no knownquantitative data representing these phenomena. However, experience hasindicated that the amount of deposition is proportional to the rate ofchange of pressure drop. Thus a flow restrictor tube of the typedescribed will tent to reduce the deposition of solids precipitated fromo the heating fluid because of the gradual changes and reduction inpressure and because of the elimination of eddy currents. Furthermore,any precipitation which may still occur will be deposited over a largerarea and will therefore not have a material effect on fluid flow. Itshould be observed that a portion of the pressure drop Obtain-ed iscaused by the known phenomena of incomplete pressure regain commensuratewith the temporary constriction of a stream of flowing fluid, while anadditional portion of the pressure drop is obtained by virtue of theincreased friction between the high velocity fluid stream and the innerwall of the throat portion 202.

Although the description has been in terms of a particular type of flowrestrictor, it should be recognized that the present invention is not solimited, and that it is intended that the invention cover therestriction of flow by any venturi-shaped flow restrictor, that termbeing herein used as applying to any of the class of tubes havingflaring ends connected by a constructed middle section forming a throat.Again by way of illustration and not limitation, in the unit disclosedthe length of the throat portion 202 in the flow restrictor tube 200Lhaving the longest throat is 76.5 inches while the internal throatdiameter is 0.637 inch. Thus, it can be seen in this instance that themajor portion of the pressure drop is due to friction of the fluidflowing through the throat. The total pressure drop imposed by the flowrestrictor tube is 112 p.s.i. For comparison purposes, the flowrestrictor tube affording the least amount of flow restriction has athroat diameter of 0.810 inch and a throat length of 4 inches, resultingin a pressure drop of 10 p.s.i. Thus it can be seen that great latitudeis available in the design of flow restrictor tubes of the typedisclosed. For purposes of simplicity it is preferred to limit thenumber of different throat diameters used on a single installation totwo or three, and to obtain the desired pressure drop by varying thethroat length. It should be also noted in FIG. 4 that some of the supplypipes 109 are not provided with flow restrictor tubes. These supplypipes serve the higher heat absorption flow circuits where none or onlya small flow restriction is required.

In conjunction with the use of variable orifice type valves as discussedabove, it has been found that no deposition problems are experienced solong as the pressure drop across the valve is maintained below about 30p.s.i. Therefore, in order to compensate for any errors in estimatingthe flow through the various circuits, variable orifice valves 205 havebeen installed in those flow circuits requiring the largest flowrestriction. Thus, for example, a valve 205L is disposed upstream of theflow restrictor tube 200L in the supply pipe 109L. In the event that theestimated flow proves to be incorrect, or in the event, for example,that slag accummulations in some portion of the furnace cause a radicalchange in the fluid flow requirements of any circuit, the flow may beadjusted accordingly by means of the valves 205.

Thus in practice the venturi-shaped flow restrictor tubes 200, sizedaccording to calculated expected flow requirements in the variouscircuits, are installed in at least some of the supply pipes 109 andprovide the major flow inhibiting means for compensating for theinequality of heat absorption in the fluid heating circuits. Inaddition, the variable orifice valves 205 are installed in some of thesupply pipes 109, in series with the flow restrictor tubes 200 to affordfine adjustment so that errors in estimated flow may be compensated foras well a to allow adjustments required as a result of a change inconditions within the furnace.

While in accordance with the provisions of the statutes there isillustrated and described herein a specific em bodiment of theinvention, those skilled in the art will understand that changes may bemade in the form of the invention covered by the claims, and thatcertain features of the invention may sometimes be used to advantagewithout a corresponding use of the other features.

The claims are:

1. In a forced circulation fluid heating unit, walls defining a furnacechamber, burner means supplying high temperature heating gases to saidchamber for flow therethrough, at least one of said Walls including arow of upwardly extending contiguous 'upflow tubes arranged for parallelflow of fluid therethrough and grouped into a plurality of parallel flowcircuits some of which are arranged to absorb less heat than others, afluid distributor vessel, means for supplying a vaporizable fluid tosaid vessel, a plurality of conduits connecting said vessel for parallelflow of said fluid to the inlet ends of said upflow tubes and throughsaid circuits, each of said conduits supplying said fluid to a pluralityof said upflow tubes, and means for inhibiting the flow of fluid inthose of said circuits having a relatively low heat absorption rate tocompensate for the inequality of heat absorption in said circuitsincluding flow restrictors disposed in at least some of said conduits,each of said flow restrictors having means forming gradually diminishingand increasing transverse flow are-as to avoid abrupt changes ofpressure therein.

2. In a forced circulation fluid heating unit, walls defining a furnacechamber, burner means supplying high temperature heating gases to saidchamber for flow therethrough, a row of upwardly extending contiguousupflow tubes disposed in said walls and arranged for parallel flow offluid therethrough and grouped into a plurality of parallel flowcircuits some of which are arranged to absorb less heat than others, afluid distributor vessel, means for supplying a vaporizable fluid tosaid vessel, a plurality of conduits connecting said vessel for parallelflow of said fluid to the inlet ends of said upflow tubes and throughsaid circuits, each of said conduits supplying said fluid to a pluralityof said upflow tubes, and means for effecting a pressure drop andinhibiting the flow of fluid in those of said circuits having arelatively low heat absorption rate to compensate for the inequality ofheat absorption in said circuits including flow restrictors disposed inat least some of said conduits, each of said flow restrictors includingmeans forming gradually diminishing and increasing transverse flow areasand an elongated throat portion to avoid abrupt changes of pressure insaid flow restrictor, said throat portion being sized to effect asubstantial portion of the total pressure drop across said flowrestrictor by friction between the inner wall of said throat portion andthe fluid flowing therethrough.

3. In a forced circulation fluid heating unit, walls defining a furnacechamber, burner means supplying high temperature heating gases to saidchamber for flow therethrough, a row of upwardly extending contiguousupflow tubes disposed in said Walls and arranged for parallel flow offluid therethrough and grouped into a plurality of parallel flowcircuits some of which are arranged to absorb less heat than others, afluid distributor vessel, means for supplying to said vessel avaporiza'ble fluid containing contaminants which tend to be separatedfrom the fluid upon the occurrence of abrupt pressure changes, aplurality of conduits connecting said vessel for parallel flow of saidfluid to the inlet ends of said upflow tubes and through said circuits,each of said conduits supplying said fluid to a plurality of said upflowtubes, and means for inhibiting the flow of fluid in those of saidcircuits having a relatively low heat absorption rate to compenate forthe inequality of heat absorption in said circuits including flowrestrictors disposed in at least some of said conduits, each of saidflow restrictors having means forming gradually diminishing andincreasing flow areas to avoid abrupt changes of pressure therein andthereby inhibit deposition of said contaminants therein.

4. In a forced circulation fluid heating unit, walls defining a furnacechamber, burner means supplying high temperature heating gases to saidchamber for flow therethrough, a row of upwardly extending contiguousupflow tubes disposed in said walls in heat exchange relation to saidheating gases and arranged for parallel flow of fluid therethrough andgrouped into a plurality of parallel flow circuits some of which arearranged to absorb less heat than others, a spherical distributorvessel, means for supplying to said vessel a liquid/vapor mixturecontaining contaminants which tend to be separated from the mixture uponthe occurrence of abrupt pressure changes; a plurality of. conduitsdisposed outside of said chamber and connecting said vessel for parallelflow of said mixture to the inlet ends of said upflow tubes and throughsaid circuits, each of said conduits supplying said mixture to aplurality of said upflow tubes, said vessel being symmetricallyconstructed about its vertical axis to effect the qualitativeapportionment of said mixture to said conduits, and means for effectinga pressure drop and inhibiting the flow of said mixture in those of saidcircuits having a relatively low heat absorption rate so that thequantitative flow of said mixture in each of said circuits issubstantially proportional to its heat absorption rate to compensate forthe inequality of heat absorption in said circuits, said last named meanincluding venturi-shaped flow restrictors disposed in .at least some ofsaid conduits intermediate their ends, each of said flow restrictorsincluding means forming gradually diminishing and increasing transverseflow areas to avoid abrupt changes of pressure and thereby inhibitdeposition of said contaminants in said flow restrictors, each of saidflow restrictors having an elongated throat portion which is sized toeffect a substantial portion of the total pressure drop by frictionbetween the inner wall of said throat portion and the mixture flowingtherethrough.

References Cited by the Examiner UNITED STATES PATENTS 1,805,236 5/1931Connet 138-40 1,988,659 1/1935 La Mont 122406 2,175,013 10/ 1939 Blizard122-406 2,249,469 7/ 1941 Gray 138-40 2,781,028 2/1957 Armacost 122-4062,994,308 8/1961 Stabenon 122-406 3,125,995 3/1964 Koch 122-406 FOREIGNPATENTS 1,191,590 10/1959 France.

549,971 10/ 1930 Germany.

636,769 5/1950 Great Britain.

663,892 12/ 1951 Great Britaim- 786,325 11/1957 Great Britain.

PERCY L. PATRICK, Primary Examiner. MEYER PERLIN, Examiner.

1. IN A FORCED CIRCULATION FLUID HEATING UNIT, WALLS DEFINING A FURNACECHAMBER, BURNER MEANS SUPPLYING HIGH TEMPERATURE HEATING GASES TO SAIDCHAMBER FOR FLOW THERETHROUGH, AT LEAST ONE OF SAID WALLS INCLUDING AROW OF UPWARDLY EXTENDING CONTIGUOUS UPFLOW TUBES ARRANGED FOR PARALLELFLOW OF FLUID THERETHROUGH AND GROUPED INTO A PLURALITY OF PARALLEL FLOWCIRCUITS SOME OF WHICH ARE ARRANGED TO ABSORB LESS HEAT THAN OTHERS, AFLUID DISTRIBUTOR VESSEL, MEANS FOR SUPPLYING A VAPORIZABLE FLUID TOSAID VESSEL, A PLURALITY OF CONDUITS CONNECTING SAID VESSEL FOR PARALLELFLOW OF SAID FLUID TO THE INLET ENDS OF SAID UPFLOW TUBES AND THROUGHSAID CIRCUITS, EACH OF SAID CONDUITS SUPPLYING SAID FLUID TO A PLURALITYOF SAID UPFLOW TUBES, AND MEANS FOR INHIBITING THE FLOW OF FLUID INTHOSE OF SAID CIRCUITS HAVING A RELATIVELY LOW HEAT ABSORPTION RATE TOCOMPENSATE FOR THE INEQUALITY OF HEAT ABSORPTION IN SAID CIRCUITSINCLUDING FLOW RESTRICTORS DISPOSED IN AT LEAST SOME OF SAID CONDUITS,EACH OF SAID FLOW RESTRICTORS HAVING MEANS FORMING GRADUALLY DIMINISHINGAND INCREASING TRANSVERSE FLOW AREAS TO AVOID ABRUPT CHANGES OF PRESSURETHEREIN.